StructuralSteel结构钢.ppt
Structural Steel钢结构,教学目标,了解钢材作为结构材料的优缺点了解不同性能的结构钢的应力-应变关系 熟悉各种描述结构钢性能、特点的词汇 熟悉科技类文献的常用句型,A person traveling in the United States might quite understandably decide（清楚地断定）that steel was the perfect structural material.He or she would see an endless number of（无穷无尽的）steel bridges,buildings,towers,and other structures comprising,in fact,a list too lengthy to enumerate（列举）.After seeing these numerous steel structures this traveler might be quite surprised to learn（了解）that steel was not economically made in the United States until late in the nineteenth century and the first wide-flange beams were not rolled（轧制）until 1908.,Advantages of Steel as a Structural Material钢材作为结构材料的优点,一位在美国旅行的人可能会很清楚地断定钢材是理想的结构材料。他或她将看到无穷无尽的钢桥、钢的建筑物、钢塔以及其它的钢结构，实际上包含的名单太长了以至于无法列举。在观看了这些数量众多的钢结构后，这位旅行者可能会非常惊讶地了解到在美国直到19世纪后期钢材才被经济地制造，并且直到1908年才轧制出第一根宽翼缘的梁。,The assumption of the perfection（完美）of this metal,perhaps the most versatile（通用的）of structural materials,would appear to be even more reasonable（合理的）when its great strength,light weight,ease of fabrication（制作）,and many other desirable（理想的）properties are considered（考虑）.These and other advantages of structural steel are discussed in detail（详细地）in the following paragraphs.当考虑这种金属具有高强、轻质、易制作以及很多其它理想的性能时，认为其完美的假定甚至显得更合理，或许它是最通用的建筑材料。这些和其它的结构钢的优势将在下面的段落中详细地讨论。,High StrengthThe high strength of steel per unit of weight means that structure weights will be small.This fact is of great importance for long-span bridges,tall buildings,and structures having poor foundation conditions（薄弱地基条件）.高强度钢材每单位重量的高强度意味着结构的重量将是小的。这个事实对大跨的桥梁、高层建筑以及有着薄弱地基条件的结构具有重要意义。,UniformityThe properties of steel do not change appreciably（明显地）with time,as do those of reinforced-concrete structure.一致性钢材的性能随时间的变化不明显，而钢筋混凝土结构的性能则随时间变化。,ElasticitySteel behaves closer to（更接近于）design assumptions than most materials because it follows（遵循）Hookes law up to fairly high stresses.The moments of inertia（惯性矩）of a steel structure can be definitely（确切地）calculated,while the value obtained for a reinforced-concrete structure are rather indefinite（不确切）.弹性比起大多数材料，钢材的运行更接近于设计的假定，因为它直到相当高的应力仍然遵循虎克定理。一个钢结构的惯性矩可以确切地被计算，而钢筋混凝土结构得到的该值是非常得不确切。,PermanenceSteel frames that are properly maintained（维护）will last indefinitely（长期地）.Research on some of the newer steels indicates that under certain conditions no painting（涂漆）maintenance whatsoever（一点也不）will be required.持久性适当维护的钢框架将会长期地维持下去。对一些较新钢材的研究显示在某些条件下，一点也不需要涂漆的维护。,DuctilityThe property of a material by which it can withstand extensive（大量的）deformation without failure under high tensile stresses is said to be its ductility.When a mild（低碳）or low-carbon structural steel member is being tested in tension（受拉测试）,a considerable（相当大的）reduction in cross section and a large amount elongation will occur at the point of failure before the actual fracture occurs.A material that does not have this property is generally unacceptable and is probably hard and brittle and might break if subjected to a sudden shock（冲击）.延性材料在高的拉应力下能承受大的变形而不破坏的性能被称为是延性。当一根低碳结构钢构件正在进行受拉测试时，在实际的断裂发生之前，在破坏点处将会发生横断面的大大缩小以及产生很大的伸长。不具有这种性能的材料通常是不被接受的，并且该材料可能是坚硬和脆性的，如果承受一个突然的冲击，可能会断裂。,In structural members under normal（正常的）loads,high stress concentrations develop（形成）at various points.The ductile nature of the usual（普通的）structural steels enables them to yield locally at those points,thus preventing premature（过早的）failures.A further（另一个）advantage of ductile structures is that when overloaded their large deflections give visible（明显的）evidence of impending failure（即将破坏的迹象）(sometimes jokingly referred to as“running time”“逃跑时间”).在正常荷载下的结构构件中，在不同的点上会形成高应力集中。普通结构钢延性的特点使它们在这些点上局部地屈服，这样阻止了过早的破坏。延性结构的另一个优势是当超载时，它们的大变形显示出明显的即将破坏的迹象（有时候戏称为“逃跑时间”）。,In Fracture ToughnessStructural steels are tough（坚韧的）;that is,they have both strength and ductility.A steel member loaded until it has large deformations will still be able to withstand large forces.This is a very important characteristic because it means that steel members can be subjected to large deformations during fabrication and erection（安装）without fracture thus allowing them to be bent,hammered,sheared（剪切）,and have holes punched（冲孔）in them without visible damage.The ability of a material to absorb energy in large amounts is called toughness”).断裂韧性结构钢是坚韧的；也就是说，它们既有强度又有延性。一根钢构件加荷至大变形时将仍然能够承受大的力。这是非常重要的特性，因为这意味着钢构件在制作和安装期间能承受大的变形而不断裂这就允许它们被弯曲、锤击、剪切以及在上面冲孔而没有明显的损害。材料吸收大量能量的能力称为韧性。,Additions to Existing StructuresSteel structures are quite well（相当）suited to having additions made to them.New bays（节间）or even entire（整个）new wings（翼）can be added to existing steel frame buildings,and steel bridges may often be widened.在对已有结构的添加钢结构相当适合对其自身进行添加。新的节间、甚至整个新的翼能被添加到已有的钢框架建筑上，因此钢桥常常可以被加宽。,MiscellaneousSeveral other important advantages of structural steel are:(a)ability to be fastened together by several simple connection devices（方法）including welds and bolts;(b)adaptation to prefabrication（预制）;(c)speed of erection;(d)ability to be rolled into a wide variety of（各种各样的）sizes and shapes;(e)fatigue strength;(f)possible reuse（再利用）after a structure is disassembled（分解）,and(g)scrap value（残余价值）,even though not reusable（可再利用的）in its existing form.Steel is the ultimate recyclable（可循环的）material.其它结构钢其它的一些重要优点是：（a）能够通过一些简单的连接方法包括焊缝和螺栓将其连接起来；（b）适合预制；（c）安装速度；（d）能够轧制成各种各样的尺寸和形状；（e）疲劳强度；（f）在结构被分解后可能的再利用；以及（g）残余价值，即使不能以其已有的形式被再利用。钢材是最终可循环的材料。,Disadvantages of Steel as a Structural Material钢材作为结构材料的缺点,In general,steel has the following disadvantages.通常，钢材具有下列缺点。,Maintenance CostsMost steels are susceptible to（易受.影响）corrosion（锈蚀）when freely（直接地）exposed to air and water and must therefore be periodically（定时地）painted.The use of weathering（风化）steels,in suitable design applications,tends to eliminate（避免）this cost.维护费用大多数钢材在直接暴露于空气和水中时容易锈蚀，因此必须被定时地涂漆。在合适的设计应用中，采用风化的钢材往往能避免该费用。,Fireproofing CostsAlthough structural members are incombustible（不燃性的）,their strength is tremendously（惊人地）reduced at temperatures commonly（通常）reached in fires when the other materials in a building burn.Many disastrous fires（火灾）have occurred in empty buildings where the only fuel（燃料）for the fires was the buildings themselves.Furthermore,steel is an excellent（极好的）heat conductor（导热体）,nonfireproofed（不耐火）steel members may transmit enough heat from a burning section or compartment（隔间）of a building to ignite（使.燃烧）materials with which they are in contact in adjoining sections of the building.As a result of these facts the steel frame of a building may have to be protected by materials with certain insulating characteristics,or the building may have to include a sprinkler system（自动喷水灭火系统）if it is to meet the building code requirements of the locality in question（正被讨论的地区）.,防火费用尽管结构构件具有不燃性，但它们的强度通常在火中达到的温度时会惊人地减少，而建筑中的其它材料则会燃烧。很多火灾发生在空的建筑物中，这里着火的唯一燃料是建筑物自己。而且，钢材是一个极好的导热体，不耐火的钢构件可以将一个房屋燃烧的区域或隔间的足够的热量传送而使邻近建筑区域内与之接触的材料燃烧。根据这些现象，建筑物的钢框架可能不得不采用具有一定绝热性能的材料加以保护，或者该建筑物不得不包含一个自动喷水灭火系统，假如它符合正被讨论地区的建筑规范的要求。,Susceptibility to BucklingThe longer and more slender the compression members,the greater the danger of buckling.As previously indicated,steel has a high strength per unit of weight,but when used for steel columns is not very economical sometimes because considerable material has to be used merely to stiffen（加强）the columns against buckling.容易屈曲受压构件越是长和细的，其屈曲的危险越大。正如前面所显示的，钢材单位重量的强度高，但是用于钢柱时有时并不经济，因为不得不采用相当多的材料仅仅是为了加强柱子以防屈曲。,FatigueAnother undesirable（不受欢迎的）property of steel is that its strength may be reduced if it is subjected to a large number of stress reversals（反向应力）or even to a large number of variations of tensile stress.(We have fatigue problems only when tension is involved.)The present practice（习惯）is to reduce the estimated strengths of such members if it is anticipated（预测）that they will have more than a prescribed（规定的）number of cycles of stress variation.疲劳钢材另一个不受欢迎的特性是如果它承受大量的反向应力或者甚至承受大量的拉应力的变化，则它的强度可能会降低。（只有包括拉力时我们才有疲劳问题。）目前的习惯是如果预测到会有超过规定数量的应力变化的循环次数，则应降低该构件的估计强度。,Brittle FractureUnder certain conditions steel may lose its ductility,and brittle fracture may occur at places of stress concentration.Fatigue-type（疲劳型）loadings and very low temperatures aggravate（加重）the situation.脆性断裂某些条件下，钢材可能失去它的延性，并且可能在应力集中处发生脆性断裂。疲劳型荷载和非常低的温度加重了这种情形。,Stress-strain Relationships in Structural Steel结构钢中的应力-应变关系,In order to understand the behavior of steel structures,it is absolutely essential for the designer to be familiar with the properties of steel.Stress-strain diagrams present valuable（有价值的）information necessary to understand how steel will behave in a given situation.Satisfactory steel design methods cannot be developed（得到）unless complete information is available concerning the stress-strain relationships of the material being used.为了解钢结构的性能，设计人员绝对有必要熟悉钢材的性能。应力-应变图显示了有价值的信息，它对理解钢材在一个给定的条件下如何运行是必要的。除非获得关于正被使用的材料的应力-应变关系的完整的信息，否则就不能得到满意的钢材设计方法。,If a piece of（一段）ductile structural steel is subjected to a tensile force it will begin to elongate.If the tensile force is increased at a constant（不变的）rate,the amount of elongation will increase constantly（不断地）within certain limits（范围）.In other words,elongation will double when the stress goes（变成）from 6,000 to 12,000 psi(pounds per square inch).When the tensile stress reaches a value roughly（大约）equal to one-half of the ultimate strength of the steel,the elongation will begin to increase at a greater rate without a corresponding increase in the stress.如果一段延性结构钢承受一个拉力，它便开始伸长。如果该拉力以一个不变的速度增加，则伸长值将在一定的范围内不断地增加。换句话说，当应力从6000磅每平方英寸变成12000磅每平方英寸时，伸长将加倍。当拉应力达到的值约等于钢材极限强度的一半时，伸长开始以一个较大的速度增加，而应力没有相应的增加。,The largest stress for which Hookes law applies（适用）or the highest point on the straight-line portion of the stress-strain diagram is the proportional limit（比例极限）.The largest stress that a material can withstand without being permanently（永久地）deformed is called the elastic limit（弹性极限）.This value is seldom actually measured and for most engineering materials including structural steel is synonymous with（与.意思相同）the proportional limit.For this reason the term proportional elastic limit is sometimes used.虎克定理适用的最大应力或者应力-应变图的直线部分的最高点是比例极限。一种材料能承受的且不发生永久变形的最大应力称为弹性极限。这个值实际上很少被测出，而且对包括结构钢在内的大多数工程材料来说，该值与比例极限同义。因此，术语弹性比例极限有时被采用。,The stress at which there is a decided（明显的）increase in the elongation or strain without a corresponding increase in stress is said to be the yield stress.It is the first point on the stress-strain diagram where a tangent to（.的切线）the curve is horizontal.The yield stress is probably the most important property of steel to the designer,as so many（很多的）design procedures are based on this value.Beyond the yield stress there is a range in which a considerable increase in strain occurs without increase in stress.The strain that occurs before the yield stress is referred to as the elastic strain;the strain that occurs after the yield stress,with no increase in stress,is referred to as the plastic strain.Plastic strains are usually from 10 to 15 times the elastic strains.,在某个应力上伸长或应变有明显的增加而应力没有相应的增加，该应力称为屈服应力。它是应力-应变图上其所在曲线的切线是水平的第一个点。对设计者来说，屈服应力可能是钢材的最重要的特性，因为很多的设计方法是基于该值的。超过屈服应力后，有一段范围中应变发生相当大的增加而应力没有增加。屈服应力发生之前的应变称为弹性应变；屈服应力发生之后的没有应力增加的应变称为塑性应变。塑性应变通常是弹性应变的10到15倍。,Yielding of steel without stress increase may be thought to be a severe disadvantage when（而）in actuality（实际上）it is a very useful characteristic.It has often performed the wonderful（出色的）service of preventing failure due to omissions（忽略）or mistakes on the designers part（在.方面）.Should（倒装，表示如果.）the stress at one point in a ductile steel structure reach the yield point,that part of the structure will yield locally without stress increase,thus preventing premature failure.This ductility allows the stresses in a steel structure to be readjusted（重新调整）.Another way of describing this phenomenon is to say that very high stresses caused by fabrication,erection,or loading will tend to equalize（使.平衡）themselves.It might also be said that a steel structure has a reserve（储备）of plastic strain that enables it to resist overloads and sudden shocks.If it did not have this ability,it might suddenly fracture,like glass or other vitreous（玻璃质的）substances.,没有应力增加的钢材屈服可能被认为是一个严重的缺点，而实际上它是一个非常有用的特性。它已常常有效地防止了由于设计者方面的忽略或错误而导致的破坏。如果延性钢结构中的某一点应力达到屈服点，则该结构部分将局部屈服而没有应力的增加，这样便阻止了过早破坏。延性允许钢结构中的应力进行再调整。另一种描述这个现象的方法是说由制造、安装或加荷引起的非常高的应力将往往使它们自身得到平衡。也可以说钢结构具有塑性应变的储备以使它能抵抗超载和突然的冲击。如果它不具有该能力，它可能会象玻璃或其它玻璃质的物质那样突然断裂。,Following（在.之后）the plastic strain there is a range in which additional（额外的）stress is necessary to produce additional strain.This is called strain-hardening.This portion of the diagram is not too important to todays designer because the strains are so large.A familiar stress-strain diagram for mild or low-carbon structural steel is shown in Fig.10-1.Only the initial part of the curve is shown here because of the great（很多的）deformation which occurs before failure.At failure in the mild steels the total strains are from 150 to 200 times the elastic strains.The curve will actually continue up to its maximum stress value and then“tail off”（减小）before failure.A sharp（急剧的）reduction in the cross section of the members takes place(called“necking”颈缩)followed by failure.,塑性应变后有一段范围，在该段范围内要产生额外的应变必须有额外的应力。这段称为应变硬化。这部分图对今天的设计者来说不太重要，因为应变太大了。一张熟悉的低碳结构钢的应力-应变图如图10-1所示（图10-1是典型的低碳结构钢在室温下的应力-应变图）。这里只显示了曲线的初始部分，因为很多的变形发生在失效之前。在低碳钢失效时，总的应变是弹性应变的150到200倍。曲线实际上将继续至它的最大应力值，然后在破坏之前应力变小。紧接着构件的破坏，其横截面会发生急剧的减小（称为颈缩）。,The stress-stain curve of Fig.10-1 is typical of（对.是典型的）the usual ductile structural steel and is assumed to be the same for members in tension or compression.(The compression members must be stocky（粗短的）because slender compression members subjected to compression loads tend to bend laterally（侧向地）,and their properties are greatly affected by the bending moments so produced.)The shape of the diagram varies with the speed of loading,the type of steel,and the temperature.One such variation is shown in the figure by the dotted line（虚线）marked upper yield（标明屈服上限）.This shape stress-strain curve is the result when a mild steel has the load applied rapidly,while the lower yield is the case for slow loading.,图10-1中的应力-应变曲线对一般的延性结构钢来说是典型的，并假定构件在受拉和受压时是相同的。（受压构件必须是粗短的，因为细长的受压构件承受压力时往往会侧向弯曲，且它们的性能大大地受这样形成的弯矩的影响。）图的形状随加荷的速度、钢材的类型和温度而变化。这样的一个变化通过一根标明屈服上限的虚线显示在图中。该应力-应变曲线的形状是低碳钢快速加载的结果，而屈服下限是缓慢加载的情况。,You should note that the stress-strain diagram of Fig.10-1 was prepared for（为.而准备）mild steel at room temperature.Steels(particularly those with rather high carbon contents)may actually increase a little in strength as they are heated to a temperature of about 700(Fahrenheit).As temperatures are raised into the 800 to 1000 range,steel strengths are drastically（急剧地）reduced,and at 1200 they have little strength left.你应该注意到图10-1中的应力-应变图是用于室温下的低碳钢。当钢材（特别是那些有着相当高的碳含量的钢材）被加热至温度大约为700时，其强度实际上可能会增加一些。当温度被升高至800 到 1000的范围时，钢材的强度急剧地减少，而在1200时几乎不再有强度。,Typical ratios（比值）of yield stresses at high temperatures to yield stresses at room temperatures are approximately 0.77 at 800,0.63 at 1000,and 0.37 at 1200.Temperatures in these ranges can be easily reached in steel members during fires,in localized areas of members when welding is being performed,in members in foundries（铸造车间）over open flame（明
